The present invention relates to non-linear optical frequency conversion systems, and in particular to the design of optical frequency mixers.
Non-linear optical crystals have been used for optical frequency mixing. FIG. 1 shows two laser frequencies xcfx891, xcfx892 passing through a nonlinear optical (xe2x80x9cNLOxe2x80x9d) crystal 100. The non-linear effect of the medium synthesizes a laser beam with a third frequency, xcfx893 where xcfx893=xcfx891xc2x1xcfx892.
This nonlinear process is enhanced when the refractive index of the crystal is properly matched. This is called the phase matching condition. This proper match can be controlled by angle tuning of the crystal orientation with respect to the three laser beams xcfx891, xcfx892 and xcfx893. Phase matching can also be carried out by changing the temperature, or by applying a difference electric field, etc.
An optical frequency mixer which has xcfx891=n*xcfx89, xcfx892=m*xcfx89, and xcfx893=(n+m)xcfx89, where n,m are integers are greater than 0, is called a harmonic generator.
One example uses the fundamental of the Nd:YAG laser 1064 nm (xcfx891), the second harmonic at 532 nm (green, xcfx89532=xcfx891064+xcfx891064), and the fourth harmonic at 266 nm (Ultra-Violet, UV, xcfx89266=xcfx89532+xcfx89532).
Optical frequency mixers allow a single laser source to be used for multiple different operations. For example, the fundamental output of a Nd:YAG laser is at 1064 nm. This frequency is not properly tuned to drill though many metals and plastics effectively. The fourth harmonic of 1064 nm, at 266 nm, however, allows the laser pulses to drill smooth holes with great ease in these materials.
Harmonics of lasers can also be used to pump tunable lasers, and enables covering a much broader spectral range. Optical frequency mixing research often attempts to generate higher power harmonics of solid state lasers such as Nd:YAG lasers in the deep UV range, e.g.  less than 300 nm down to 190 nm. These deep UV sources are also used for microlithography. Deep UV sources permit small features e.g. 0.1 xcexcm to be formed. Deep UV sources are also used for many industrial and scientific applications including inkjet nozzle drilling, laser eye surgery, trace gas monitoring and dissociation and others.
Although gas lasers could be used to generate a high power UV source, their size and the power consumed can be less favorable than current solid state lasers.